During label manufacture, a laminate of a face stock, pressure-sensitive adhesive layer and a release liner is passed through apparatus which convert the laminate to yield commercially useful labels and label stock. The processes involved in the converting operation include printing, die cutting and matrix stripping to leave labels on a release liner, butt cutting of labels to the release liner, marginal hole punching, perforating, fan folding, guillotining and the like.
Die and butt cutting involve cutting of the laminate to the face of the release liner. Other procedures involve cutting clean through the label laminate and include hole punching, perforating and guillotining.
The cost of converting a laminate into a finished product is a function of the speed at which the various processing operations occur. While the nature of all layers of the laminate can impact cost of convertibility, the adhesive layer has been the greatest limiting factor in ease and cost of convertibility. This is in consequence of its viscoelastic nature which hampers precise and clean penetration of a die in die cutting operations and promotes adherence to cutting blades and the like in cutting operations. Stringiness of the adhesive also impacts matrix stripping operations which follow die cutting operations.
Achieving good convertibility does not, by necessity, coincide with achieving excellent adhesive performance. Adhesives must be formulated to fit needs and important properties include peel adhesion, tack, shear, viscosity at various temperatures and the like. Good general purpose adhesives may exhibit poor convertibility simply because the adhesive is difficult to cleanly cut. Such an adhesive may stick to a die or blade. In label manufacture, die cutting and matrix stripping operations by necessity occur at a variety of speeds in the range of 0 to 300 meters per minute. Within the range, an adhesive may provide regions where a matrix will break despite the fact that successful matrix stripping can occur at speeds on either side of the region. One goal is to provide adhesive systems wherein the adhesive can be cleanly cut and the matrix stripped over the entire range of operating speeds, as shown as a cutting frequency in attached FIGS. 1 and 2.
Natural and synthetic elastomers containing polybutadiene segments and polyisoprene segments are not miscible with each other. It is known in the art, however, to mix immiscible elastomeric polymers based on polybutadiene and polyisoprene. Homopolymers are more difficult to employ than block copolymers. With block copolymers, while the midblocks are immiscible, the end blocks of polystyrene form one common domain making the mixture stable without major problems of incompatibility such as separation of phases with time.
Additives to mixtures based on polyisoprene and polybutadiene vary in their compatibility with the polybutadiene or polyisoprene portions of the elastomer. While preferentially soluble in either the polyisoprene or the polybutadiene, normally there is some degree of compatibility with both components. Of the additives, those which tend to have no impact or increase glass transition temperature are regarded in the art as tackifiers and those which tend to lower glass transition temperature as plasticizers. Additives may be preferentially soluble in the polystyrene portion and may act as a reinforcing agent.
U.S. Pat. No. 4,125,665 to Bemmels, et al. is directed to a pressure-sensitive adhesive for use as a sealing tape in applications such as closure for juice and beverage cans having a top with a preformed aperture for liquid dispensing. The pressure-sensitive adhesive layer contains a relatively high melting point tackifier resin and a relatively low molecular weight styrene block associating bonding resin. Example 2 in Table A of the '665 patent discloses a mixture of Kraton 1107, a styrene-isoprene-styrene (SIS)/styrene-isoprene (SI) block copolymer and Kraton 1102, a styrene butadiene (SB)/styrene-butadiene-styrene (SBS) block copolymer, both manufactured by Shell, along with Wingtack 95, a C.sub.5 or C.sub.6 normally solid tackifier as taught in U.S. Pat. No. 3,577,398 assigned to Goodyear Tire and Rubber Company and incorporated herein by reference, and a styrene block associating bonding resin. In the absence of the styrene block associating bonding resin, the mixture displayed, according to the patentees, poor adhesion to steel and the addition of the styrene associating material was necessary to overcome the deficiency.
U.S. Pat. No. 4,080,348 to Korpman discloses in Example 2 of Table A an SI block copolymer, an SBS block copolymer and, as a tackifier, Wingtack 76. The weight ratio of the butadiene containing block copolymer to the isoprene containing block copolymer is 0.25:1. We have found that such mixtures only show one glass transition temperature to be present, that being the one predominantly contributed by the isoprene containing block copolymer.
Other patents of interest are U.S. Pat. No. 3,880,953 to Downey, U.S. Pat. No. 3,509,239 to Tindall, U.S. Pat. No. 4,060,503 to Feeney, et al. and U.S. Pat. No. 3,932,328 to Korpman which employs as a hot-melt adhesive mixture SIS block copolymer tackified with a blend of normally solid and normally liquid tackifiers consisting essentially of polymerized structures derived from aliphatic dienes and mono-olefins of 5 or 6 carbon atoms (Wingtack 95 and Wingtack 10 manufactured by Goodyear); and U.S. Pat. No. 4,411,954 to Butch III, et al.